A lens is a device usually formed from a piece of shaped glass or plastic that causes light to either converge and concentrate, or to diverge. One important use of lenses is as a prosthetic for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. Other uses are in imaging systems such as a monocular, binoculars, telescope, spotting scope, telescopic gun sight, theodolite, microscope, and camera (photographic lens).
Lenses do not form perfect images; there is always some degree of distortion or aberration introduced by the lens that causes the image to be an imperfect replica of the object. Thus, aberrations result when the optical system misdirects some of the object's rays. There are several types of aberrations that can affect image quality. Some aberrations occur when electromagnetic radiation of one wavelength is being imaged (monochromatic aberrations), and others occur when electromagnetic radiation of two or more wavelengths is imaged (chromatic aberrations).
Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens).
Since the focal length ƒ of a lens is dependent on the refractive index n, different wavelengths of light will be focused at different locations. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens; and transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength). Longitudinal and lateral chromatic aberration of a lens is seen as “fringes” of color around the image, because each color in the optical spectrum cannot be focused at a single common point. For example, eyeglass wearers, with strong myopic correction, can experience color spreading in the periphery of the eyeglass lenses. Although the brain will mask perception of these colored fringes after a period of adaptation, their deleterious effect on visual acuity remain.
Because the distortion introduced by aberrations into an optical system significantly degrades the quality of the images on the image plane of such system, there are advantages to the reduction of those aberrations. Various techniques are often used to reduce the aberrations. One such technique involves the use of a wavefront aberrator.
Wavefront aberrators are particularly useful in eye glasses or contact lenses for use in correcting human eye sight. U.S. Pat. No. 6,989,938 describes one such wavefront aberrator and methods for manufacturing it. U.S. Pat. No. 6,712,466 describes eyeglass lenses having a variable index of refraction.